A programmable logic controller (also referred to as PLC) used as a control device for controlling an apparatus such as industrial machinery is sometimes multiplexed in order to ensure robustness against a failure. In a system with the control device being multiplexed, when the control device in the active system fails and then the control device in the active system is switched to a multiplexed control device, an apparatus having communicated with the control device communicates with the multiplexed control device.
In Patent Document 1, a technique is disclosed in which two programmable controllers, i.e. duplexed program controllers, each have the same transmission address set therein and an optical switching means switches the programmable controller of the active system. Thus, by using the technique shown in Patent Document 1, even when the programmable controller in the active system is switched, slave-station-side devices can continue their communication using the same addresses, regardless of the switching.